Structural transformations of quartz and berlinite AlPO4 at high pressure and room temperature: a transmission electron microscopy study

Single crystals of α-quartz and α-berlinite AlPO₄ have been compressed at high pressure and room temperature in a diamond anvil cell (DAC). The pressure-induced microstructures have been studied on recovered specimens using transmission electron microscopy. As previously reported, quartz is shown to exhibit an amorphous transition at high pressure (≈30 GPa). Under the markedly non-hydrostatic conditions of the present study, a wide mixed-phase regime in which amorphous lamellae form within the crystalline matrix is encountered at lower values of the mean stress. The amorphous lamellae are interpreted as shear lamellae. The formation of these shear lamellae as well as their habit planes are described by the evolution with pressure of shear moduli μ as computed in anisotropic elasticity. Our calculations also show instabilities at higher pressure of the elastic moduli (i.e. of the α-quartz structure) which are related to the amorphous transition. Berlinite exhibits a more ductile behavior with simultaneous dislocation activity and shear on amorphous lamellae which become pervasive at high pressure (≈10 GPa). These amorphous lamellae of berlinite do not revert to crystal when pressure is released.